1. Field of the Invention
The present invention relates to a projection optical system and a projection image display apparatus and, in particular, to technology for reducing the depths of the projection optical system and the projection image display apparatus and reducing the heights of the upper and lower portions above and below a screen while maintaining a high resolution and low distortion of an image.
2. Description of the Related Art
Recently, the need for projection television systems has been rapidly increasing. Projection television systems have features common to flat televisions, such as liquid crystal displays (LCDs) and plasma display panels (PDPs). As compared with LCDs and PDPs, projection television systems can provide a large screen at low cost because projection television systems project an image displayed on an image display element having a size of about one inch while enlarging the image. In addition, such an image display element can relatively easily support a high resolution of a full high definition (HD) signal, such as a broadcast satellite (BS) broadcasting signal or a terrestrial digital broadcasting signal.
However, projection television systems include a light source, an illumination optical system, and a projection optical system. Projection television systems further include a large rear-mounted mirror for projecting an image onto a screen. Accordingly, as shown in FIG. 38, when projection television systems project an image using a known method in which a light axis “a” passes through the center of an image display element b and the center of a screen “c”, a depth “d” and a size “e” of a portion below a screen cannot be reduced at the same time.
In order to achieve a thin size of projection television systems (reduction in size in the depth direction), some optical systems obliquely project an image onto a screen (hereinafter referred to as “oblique projection”).
For example, Japanese Unexamined Patent Application Publication No. 5-100312 (hereinafter referred to as “Patent Document 1”) describes a projection system that includes a projection optical system having a very wide-angle lens, an image display element, and a screen. The image display element and the screen are disposed so as to be offset from the light axis of the projection optical system. By using the edge portion of the angle of view of the wide-angle lens, oblique projection is achieved (refer to FIG. 39).
In addition, Japanese Unexamined Patent Application Publication No. 5-80418 (hereinafter referred to as “Patent Document 2”) describes a projection system including first and second projection optical systems. The first projection optical system forms an intermediate image from imaging light based on a light valve. The second projection optical system projects the intermediate image onto a screen while enlarging the intermediate image. The light axes of the two projection optical systems are appropriately inclined so that oblique projection is achieved (refer to FIG. 40).
Furthermore, recently, a projection optical system has been developed that achieves oblique projection by employing a reflecting surface in order to prevent the occurrence of chromatic aberration due to a wide angle of view.
PCT Japanese Translation Patent Re-Publication No. WO01/006295 (hereinafter referred to as “Patent Document 3”) describes a projection optical system that achieves oblique projection by employing an optical system mainly composed of a refractive optical system g and a convex reflecting surface f. FIG. 41 schematically illustrates the optical system described in Patent Document 3.
In addition, Japanese Unexamined Patent Application Publication No. 2002-40326 (hereinafter referred to as “Patent Document 4”) describes a projection optical system that removes chromatic aberration by employing a concave reflecting surface i, a convex reflecting surface j, a convex reflecting surface k, and a convex reflecting surface 1 disposed in this order from an image display panel h (refer to FIG. 42).
The optical systems described in the patent documents above have rotational symmetry with respect to the light axis. By employing part of an ultra-wide-angle optical system, the optical systems achieve oblique projection.
Japanese Unexamined Patent Application Publication No. 2001-255462 (hereinafter referred to as “Patent Document 5”) describes an oblique projection optical system including an off-axial optical system. In this optical system, the off-axial optical system corrects trapezium distortion. Furthermore, by forming an intermediate image between each of a plurality of reflecting surfaces m and a reflecting surface n, a projection optical system having reduced sizes of the reflecting surfaces can be achieved (refer to FIG. 43).
Japanese Unexamined Patent Application Publication No. 2004-258620 (hereinafter referred to as “Patent Document 6”) describes a projection optical system including a refractive optical system and at least one refracting surface including a concave reflecting surface. In the embodiment, part of a first optical system serving as a refractive optical system is made eccentric, and an adjustable surface is employed for a concave reflecting surface. In this way, a projection optical system with low distortion can be achieved (refer to FIG. 44).
International publication WO2006-043666 A1 (hereinafter referred to as “Patent Document 7”) describes an ultra-wide-angle optical system that has rotational symmetry and that reduces the size of a reflecting surface by forming an intermediate image using at least one concave reflecting surface AM (refer to FIG. 45).